This invention pertains to the field of analytical chemistry. More particularly, the invention pertains to liquid chromatography in combination with electrospray ionization mass spectrometry. In accordance with the method of the present invention, a plurality of sample streams eluting from a plurality of HPLC (High Pressure Liquid Chromatography) columns are coupled into a single inlet mass spectrometer.
It is known in the prior art to create multiple (array) nozzles for electrospray ionization. The known methods, in general, utilize one of three techniques for achieving separate signals from multiple spray nozzles. In one known method, the inlet is translated with respect to the nozzle xe2x80x9carrayxe2x80x9d while all of the nozzles spray simultaneously. In a second known method, the electrospray high voltage is switched on and off on each nozzle. In yet a third method, a mass spectrometer with multiple nozzles is used, each nozzle having a separate inlet. In some cases, a combination of at least two of the foregoing methods is used.
Andren et. al., in Proceedings for the 46th ASMS Conference on Mass Spectrometry and Allied Topics, Page 889, 1998, discloses a method utilizing a multiple probe spray source with two fixed spray nozzles. Each nozzle voltage can be turned on or off independently. A primary nozzle was configured to spray xe2x80x9chead onxe2x80x9d into the inlet. The secondary nozzle was fixed at an angle of 60xc2x0 to the first. The two nozzles were separated by a moderate distance. This method corresponds to the second method discussed above.
Jiang and Moini, in Analytical Chemistry 72, 20-24, 2000, disclose a multiple inlet source in which four independent spray nozzles are each provided with a separate inlet (sampling orifice) into a mass spectrometer. Spectra are obtained from the four nozzles simultaneously. This corresponds to the third method discussed above.
Hannis and Muddiman, in Journal of the American Society for Mass Spectrometry 11, 876-883, 2000, disclose a two-nozzle interface that sequentially moves each nozzle in front of a mass spectrometer inlet by electromechanical means. This corresponds to the first method discussed above.
Karger et al. U.S. Pat. No. 5,872,010, discloses an array of nozzles on a chip-based device. The nozzles are switched in voltage and translated in front of a single mass spectrometer inlet. This corresponds to a combination of the first and second methods discussed above.
Kassel et al. U.S. Pat. No. 6,066,848 discloses a linear array of nozzles in front of a single inlet. A screen device which blocks the signal from all but one of the nozzles is placed between the array and the inlet. By electro-mechanically translating the screen, signal from each of the nozzles can be obtained sequentially. All of the nozzles remain operational, i.e., xe2x80x9conxe2x80x9d, throughout the procedure.
None of the foregoing methods is completely satisfactory, however, especially where high sensitivity is required.
In accordance with the present invention, an array of a plurality of nozzles, either linear or two-dimensional, is positioned in front of the inlet of an electrospray ionization mass spectrometer. Samples for analysis are delivered through each nozzle, using known pumping systems.
The nozzles, in accordance with the present invention, when in an xe2x80x9coffxe2x80x9d mode, are held at a fractional voltage, which is between the electrical potential of the mass spectrometer inlet and the electrospray threshold value. In addition, each nozzle of the present invention is provided with a integral fluid removal tube or channel, which is separate from the channel that delivers sample material to the nozzle.
The fractional voltage enables the nozzles to be located in close proximity to each other by minimizing or eliminating the effect of the electric field of an xe2x80x9conxe2x80x9d nozzle on an adjacent xe2x80x9coffxe2x80x9d nozzle; and the separate integral fluid removal tube or channel provided to each nozzle enables a capillary wicking or the application of a vacuum suction to remove excess fluid from the nozzles.
In the method of the present invention, an array of a plurality of nozzles, which can be either linear or two-dimensional, is positioned in front of the inlet of an electrospray ionization mass spectrometer. Each of the nozzles is provided with or operated in accordance with the following two features:
In the first feature, the voltage suppled to a given nozzle is not simply switched from xe2x80x9conxe2x80x9d when charging samples to the mass spectrometer to xe2x80x9coffxe2x80x9d at the completion of the sample charge, as in the prior art. Instead, upon completion of the charge of the sample from the nozzle to the mass spectrometer, the voltage applied between the nozzle and the inlet system to the mass spectrometer, for a nozzle in the xe2x80x9coffxe2x80x9d position, is adjusted to a value that is below the electrospray threshold voltage, but above the voltage applied to the inlet to the mass spectrometer. As a result of this voltage adjustment, the generation of ion current, or signal, that would otherwise be generated in the nozzle is minimized or eliminated.
The xe2x80x9celectrospray threshold voltagexe2x80x9d is the minimum voltage at which an electrostatically generated spray with a stable cone (known as a xe2x80x9cTaylor Conexe2x80x9d) is formed. This is also known in the art as the xe2x80x9ccone jetxe2x80x9d mode. At a voltage slightly below the threshold voltage, i.e., within about 10% below the threshold voltage, transient sprays can be formed and/or large droplets which xe2x80x9cspitxe2x80x9d from the end of the nozzle can be formed. At voltages which are more than about 10% below the threshold voltage, no droplets are generated.
Thus, when a nozzle is in the xe2x80x9coffxe2x80x9d position, in accordance with the present invention, the voltage is set at a value at which the electrostatic attraction of the liquid to the counter-electrode is unable to overcome the surface tension holding the liquid together at the end of the nozzle, and therefore no liquid (droplets or spray) emits from the nozzle.
In the prior art, when a nozzle that was in an xe2x80x9coffxe2x80x9d mode located in close proximity to a nozzle that was in the xe2x80x9conxe2x80x9d mode, was allowed to float at an unspecified voltage, fluid remaining in the nozzle that was in the xe2x80x9coffxe2x80x9d mode could be affected by an electrical field generated by the xe2x80x9conxe2x80x9d nozzle and be induced into a temporary xe2x80x9conxe2x80x9d state. As a result of this, the mass spectrometer could obtain signals from more than one nozzle at the same time. This problem in the prior art limits the ultimate packing density of nozzles placed in front of a mass spectrometer inlet. In order to eliminate the induction of signal in a nozzle that is in the off mode from an adjacent nozzle that is in the xe2x80x9conxe2x80x9d mode, the individual nozzles of the prior art must be spaced further apart from each other than would otherwise be desirable.
Alternatively, the voltage of an xe2x80x9coffxe2x80x9d nozzle was held at ground, or at a potential close to that of the mass spectrometer inlet in the prior art. In this case, the electrical field surrounding an xe2x80x9conxe2x80x9d nozzle could generate ions which are then attracted to a nearby xe2x80x9coffxe2x80x9d nozzle. In order to minimize or eliminate this effect, the nozzles have to be spaced apart from each other a greater distance than would otherwise be desirable.
In addition, the phenomenon described above could, in the prior art, cause chemical contamination of the xe2x80x9coffxe2x80x9d nozzles with fluid from the xe2x80x9conxe2x80x9d nozzles.
By using the method of the present invention, wherein the xe2x80x9coffxe2x80x9d nozzles are held at a factional voltage, which is between the electrical potential of the mass spectrometer inlet and that of the electrospray threshold value, the nozzles are able to be placed in closer proximity to each other than has heretofore been possible, while minimizing or eliminating the above-described induction of transient signals between nozzles.
In a second aspect of the present invention, each nozzle is provided with an integral fluid removal tube or channel, which is separate from the channel or tube through which sample fluids are supplied to the nozzle. This fluid removal tube or channel provides a capillary wicking or active vacuum suction to remove excess fluid from the nozzle. The action of the fluid removal tube or channel is switchable between being active (on) or inactive (off). Thus, when a nozzle is brought below the electrospray threshold voltage, the action of the fluid removal channel is turned on to remove any fluid that remains in or continues to flow through that nozzle. By doing this, such remaining fluid is prevented from accumulating at the tip of the xe2x80x9coffxe2x80x9d nozzle. This, in turn, minimizes or eliminates the prior art difficulties wherein excess fluid would accumulate at a nozzle end, and swamp the nozzle array. This swamping, in turn, leads to unstable signals in the prior art, if the accumulated excess fluid comes in contact with an xe2x80x9conxe2x80x9d nozzle; and also can lead to cross-contamination of the samples in the nozzle array. In addition, the removal of excess fluid from the xe2x80x9coffxe2x80x9d nozzles, in accordance with the present invention, provides for a rapid stabilizing of a nozzle when switched from the off state to the on state.